This invention relates to a process for making low density abrasion and high temperature resistant, open/closed cellular spheres with or without a central core cavity inter and cross linked to zigzag pore conduits formed in sphere wall of desired thickness having high crush strength and forming low-to-high temperature loose fill spheres for use as catalyst carriers or for being moulded to form refractory bricks/monoliths for use in a wide range of mobile or stationary and other refractory applications.
The following expressions wherever appearing throughout this specification shall include the meaning set there against as under:
xe2x80x9cIndialite Spheresxe2x80x99xe2x80x94means low-to-high temperature loose fill and moulded macro porous spheres made by the process of this invention.
xe2x80x98PVA Aqua Solxe2x80x99 means cold water soluble inorganic binder with 2% low ash content of PVA (Poly-Vinyl-Alcohol)
xe2x80x98Burnable corexe2x80x99 means herein stated organic/inorganic filler/binder mass forming burnable core particles.
It is known that the catalyst carrier materials used for oxidation/reduction reaction possess high stability. High porosity of inert ceramic carrier enables applications as well as anchoring of any other high surface area enhancing materials such as xe2x80x98Gammaxe2x80x99 or xe2x80x98Thetaxe2x80x99 Alumina which in turn increases the degree of dispersion of the catalyst species.
These dispersions retard activity loss from agglomeration or coalescence of catalyst species during sintering and increase the availability of catalyst surface area for catalytic reactions.
The use of a wide variety of inorganic refractory metal oxides and metal oxide mixtures as catalysts supports is well known in chemical engineering and such supports are made in many forms such as loose fill packing of spheres of uniform or different sizes, rings and the like, or entire structures in reticule or honeycomb and the like form.
These supports have been utilised for carrying metal catalysts such as Pt, Pd, Rh and the like and base metal catalysts including alumina, magnesia, titania and the like either alone or any desired admixtures thereof.
Therefore, it is obviously desirable that the catalyst carrier should possess high geometric surface area, and should preferably be able to promote a degree of turbulence in order that reacting fluids or gases passing through the catalytic bed encounter as much reactive catalytic surface as possible during residence time of reaction mixture in catalyst bed.
On the other hand it is also necessary that the catalyst bed should present very little resistance to the flow of reacting fluids or gases while passing through the packed column of catalyst bed. Both the above requirements are contradictory to each other and hence an optimal combination of the two factors is necessary for achieving the best results.
It is known that catalyst carriers, in general, are inactive with decreasing level of porosity for use in conjunction with high surface area coating on which the catalyst carrier can be distributed to produce desired level of gases or fluids that come in contact with them while being circulated through said carriers in catalvst bed. To enable strong adherence of said high surface area coating, the carrier material should also have optimal pore structure and pore size distribution.
Many catalyst carriers are known in prior art, as catalvst support in the form of spheres or otherwise shaped monoliths such as honeycomb monoliths. For example, C. F. Schafer and R. C. Bedford have described an alumina based thermally stable catalyst carrier described in U.S. Pat. No. 4,31,565.
Many such patents on catalyst supports are predominant but they principally deal with active alumina. Most of the earlier references on catalvst carriers referring to xe2x80x98Cordieritexe2x80x99 composition deal with honey-comb shapes and rarely on xe2x80x98Cordieritexe2x80x99 spheres.
Numerous patents on xe2x80x98Cordieritexe2x80x99 honeycomb shapes are known in the prior art for instance:
U.S. Pat. No. 5,549,725 discloses a process for making a xe2x80x98Cordieritexe2x80x99 ceramic honeycomb for use as filter material.
U.S. Pat. Nos. 5,773,103 and 4,871,693 disclose methods of making porous xe2x80x98Cordieritexe2x80x99 ceramic formed from hollow spherical glass ceramic powders that are obtained through spray thermal decomposition of expensive organic or aqueous-organic solvents, as starting materials.
U.S. Pat. No. 4,37,044 provides a method for preparation of xe2x80x98Cordieritexe2x80x99 bead type support structure with a raspberry-like or dimpled surface having high decree of macro porosity in which colloidal silica has been used to create raspberry-like dimpled surface. Though the use of such raspberry-like sphere surface enhances the geometric surface it is prone to attrition in turbulent flow of gases and the sharp edged sphere surface tends to smoothen out, over a period of time, due to high velocity flow stream as, for example, in automotive exhaust applications in two-stroke engine vehicle.
In all the earlier references pertaining to the Cordierite catalvst carrier of any form the cordierite precursor is subjected to firing schedule of over 48 to 72 hours which includes the soaking time at the maximum temperature of 1390 to 1410xc2x0 C. This invention discloses a process wherein the firing schedule of less than an hour yielding  greater than 90% Indialite, a high temperature polymorph of cordierite, as the only principal phase in the fired body.
The principal object of this invention, therefore, is to offer a rapid process for making hollow or solid cellular macro porous xe2x80x98Indialitexe2x80x99 ceramic spheres having hitch crush strength up to 23 lbs.(point load strength) capable of extending crush strength upto 32 lbs. by homogenizing zirconia fibers in the green mass of compositions of Table-1 and burnable core mass for attaining herein stated benefits for use as loose fill catalyst carriers and also which when moulded into refractory bricks and monoliths provide high temperature insulation in refractory applications.
It is further object of this invention to provide a method for production of a highly macroporous Indialite type spherical pelleted or beaded catalyst support that is made of low cost easily available raw materials following a method of pyroprocessing schedule of less than an hour under rotary motion of the spheres in air atmosphere which enables uniform thermal treatment of the pellets and which in turn results in in-situ formation of high temperature polymorph of Cordierite (Indialite) from its precursor materials.
It is yet another object of this invention to provide a method for forming of judiciously proportioned raw materials which yields narrow size distribution of pellets which on sintering in rotary kiln gives rise to Indialite body with less than 5% volume shrinkage, while maintaining high decree of uniformly distributed pores of 1-10 micron size. Other objects are apparent from the herein described process.
The incorporation of zirconia fibers in the homogenised product mix of TABLE-1 is also found to be very effective in bonding together network cage of inter and cross linked macro pores form reinforcement for the said spheres having high crush strength upto 23 lbs psi. In certain cases, the xe2x80x98Indialitexe2x80x99 spheres reinforced with zirconia fibers favourably enhance not only their crush strength upto 32 lbs. but also enhance their catalytic activity to otherwise inert catalyst carriers.
Novel process according to this invention for making open or closed cellular macro porous xe2x80x98Indialitexe2x80x99 ceramic spheres with or without a central core cavity cross/inter linked with zigzag macro porous conduits from naturally occurring herein stated TABLE-1 composites homogenized with burnable organic/inorganic core particles forming filler/binder; sprinkling said mass over a pan-pelletizer separately wetted with  less than 2% cold water soluble PVA (Poly Vinyl Alcohol) aqua sol and rolling to form compact green phase of spheres having  less than 25% moisture dry compacting in pelletizer said spheres before air or oven drying at 90-110 deg. C. till said moisture and core particles are partially removed before sintering in a rotary kiln at temp. varying from 1350-1380 deg. C. for 45-60 minute kiln time followed by rapidly cooling down said spheres to ambient temperature before being sieved and recovering  greater than 90% sphere yield there from with  less than 5% volume shrinkage and  less than 2% cracks in said spheres having high degree of thermal shock resistance, up to 23 lbs. Crush strength and which is capable of being enhanced to 32 lbs. by addition of zircon fibers while homogenising said mass and forming loose fill catalyst spheres and/or for moulding into bricks/monoliths forming high temp. insulation for use in refractory applications having herein stated TABLES 4, 6 and 8 of product characteristics.
According to one embodiment said macro-porous sphere having there within a central cavity is made by first rolling in a pan pelletizer wetted with PVA aqua sol binder, a ball of burnable core particle of desired core cavity size followed by sprinkling said homogenized mass into said pelletizer to form spheres of desired dimension and compacting before being subjected to two oven/tray drying steps wherein in first step they are air/oven dried at about 30 deg. C. followed by second tray drying step at 90-110 deg. C. till moisture and burnable core particles are partially destroyed and which on being sintered and cooled down to ambient temperature leaving there behind a central pore cavity surrounded by a network of open zigzag macro pores in sphere wall in the manner stated in Example-1 recovers there from loose-fill xe2x80x98Indialitexe2x80x99 spheres having herein stated product characteristics.
The process also includes the step of homogenizing zircon fibers with the product mix of TABLE-1 and burnable core mass and follow the steps of Example-1 to further enhance crush strength of xe2x80x98Indialitexe2x80x99 spheres from 23 to 32 lbs.
In the accompanying SEM (Scanning Electron Microscopic) photomicrograph and XRD scan;